In Patent Literature 1, technology related to a wavefront sensor for measuring a wavefront of light waves is disclosed. In the wavefront sensor, a characteristic (for example, light intensity) is applied to light passing through each of a plurality of lenses and image data is obtained from a light receiving element such as a CCD receiving the light. A measurement spot position is calculated from this image data, a characteristic of a converging spot is detected, a reference spot position corresponding to the converging spot having the characteristic and the measurement spot position are associated, and a wavefront is calculated from the associated reference spot position and measurement spot position.